The only way to reduce body fat - barring the use of illicit substances - is to manipulate your caloric intake/output ratio so that you burn more calories than you consume. In other words, try cutting some of the junk food out of your diet, and do three to four cardio sessions every week. This will induce fat loss in areas all over the body. However, you will not lose fat from one specific area at a time.
Rather, depending on your genetic make-up, you will notice a general reduction of overall body fat that may be more pronounced in some places than others. Some people initially lose more fat in places other than their abs, so don't get discouraged if you see fat loss in parts of your body other than your stomach. Many women lose fat first in their hip/thigh region. So be patient and results will come.
As far as some special exercise that will get you a six pack, each abdominal exercise has its advantages and its drawbacks. When targeting the abs, be sure to develop a good burn through maintenance of proper form. Combine this with regular cardio and you'll be well on your way to a flatter, firmer stomach (though, because of the aforementioned reasons, a six pack might be tough to achieve).